The structural characterization of ZnS epilayers grown by hydrogen transport vapor‐phase epitaxy on (100)‐oriented GaAs substrates is reported. X‐ray‐diffraction measurements in both double‐axis (DA) and single‐axis modes were performed to determine the residual strain tensor and the strain temperature dependence of the epilayer in the range between 25 and 650 °C. From the analysis of the data obtained by DA measurements of several symmetric Bragg reflections at different azimuth angles and several asymmetric reflections recorded in different geometries, an orthorhombic distortion of the ZnS lattice was found. The crystallographic symmetry could be explained by an asymmetric distribution of the misfit dislocation density in the interface plane along the [011] and [01¯1] directions. The temperature dependence measurements of the strain tensor components between room temperature and the growth temperature (650 °C) allowed determination of the thermal misfit between ZnS and GaAs and the linear thermal‐expansion coefficient of ZnS. Finally, triple‐crystal diffractometry and secondary‐ion‐mass spectrometry were used to investigate the chemical nature of the ZnS‐GaAs interface. The results indicate the presence of an interdiffused ZnS‐GaAs interface region, whose occurrence turns out to be associated with the initial defect structure of the substrate surface before the growth. ©1995 American Institute of Physics.